1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
Conventionally, in an electro-photographic type image forming apparatus, when an image is formed on both the front face and the back face of the same sheet, since the toner image formed on the sheet is heated to be fixed, the sheet after forming the image on the front face shrinks about a few %. Therefore, when the image is formed on both faces of the sheet, the image on the back face needs to be formed considering the shrinkage of the sheet, or a problem such as the size of the image being different between the front face and the back face occurs. As one method to solve the above problem, there is a technique to control the magnification of the image by adjusting the rotating speed of the polygon motor for rotating the polygon mirror between the front face and the back face (see Japanese Patent Application Laid-Open Publication No. 2007-179005).
Moreover, in an electro-photographic type image forming apparatus, unevenness in density in the vertical scanning direction may occur depending on the rotating cycle of the photoreceptor drum, developer or the like. Usually, the image processing is performed in a line cycle. Therefore, according to a simple configuration using a counter to count up in the line cycle, the position of the photoreceptor drum, etc. in the vertical scanning direction is calculated and the density unevenness in the vertical scanning direction is corrected based on the calculated position (Japanese Patent Application Laid-Open Publication No. 2007-156192).
However, if the rotating speed of the polygon motor is changed to adjust the magnification between the front face and the back face, the line cycle changes. With this, the interval of the photoreceptor drum, etc. in the vertical scanning direction which corresponds to the line cycle changes. This causes the problem that the calculated position of the photoreceptor drum, etc. in the vertical scanning direction becomes different in the amount that the magnification is adjusted if the line cycle is merely counted up. Therefore, according to the conventional technique, when the magnification is changed, the space between the sheets needed to be sufficiently opened by, for example, waiting for the position of the photoreceptor drum, etc. to come to its reference position in the vertical scanning direction and resetting the counter, etc. Consequently, high speed processing is not possible.